Steam generating unit



S@Pt- 1, 1964 v Y* NIS. BLODGETYT l 3,146,762

STEAM GENEBAMNG UNIT Filed May 24, 1962 "2 sheets-sheet 1 United States Patent O 3,146,762 STEAM GENERATING UNIT Norman S. Blodgett, Westboro, Mass., assigner to` Riley Stoker Corporation, Worcester, Mass., a corporation f Massachusetts Filed May 24, 1962, Ser. No. 197,486 3 Claims. (Cl. 122-479) This invention relates to a steam generating unit and more particularly to apparatus arranged to regulate the temperature of superheated steam in a boiler.

In the generation of steam for use in producing electricity by means of a turbine, it is necessary that the temperature of the steam reaching the turbine be maintained at a pre-determined value with very little variation. Furthermore, in present-day steam generating units, the temperature of the steam is very close to the failure point of the metal in the superheater tube and strict regulation is necessary to assure that the temperature of the superheat does not go above the design value and cause tube failure. One of the methods commonly used for regulating the temperature of superheated steam is that of recirculation of gas through the main furnace from the back passes of the boiler. The effect of this recirculation is to cool the gases' passing over the convection surfaces and also to increase the mass ow. Generally speaking, an increase in mass flow dominates the situation and the temperature goes up, despite the fact that the temperature of the gases passing over the superheater surfaces may be lower. Because of the fact that the recirculated gas is used in considerably greater amounts at low loads (where an increase in the heat transfer to the superheater is more necessary to maintain it at the preselected Value), a number of problems are presented. First of all, since the normal products of combustion are smaller in amount at low loads, they do not mix as thoroughly with the recirculated gas which has been introduced into the furnace as at high loads. In addition, there are difficulties in maintaining ignition and in providing a suitable location for the introduction of the recirculated gas. Also, it is possible to use recirculated gas only to a limited extent to maintain the superheat temperature; in many situations this is not suflicient. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a novel apparatus for the control of superheat by combining the effects of recirculation of gas and flame placement.

Another object of this invention is the provision of a steam generating unit making use of the recirculation of gas from the back passes to the main furnace in which gas temperature from side to side of the furnace are maintained substantially uniform and slagging is reduced to a minimum.

Another object of the instant invention is to provide a superheat control in which recirculated gas is very thoroughly mixed with the ordinary products of combustion before passage through the convection passes of the boiler.

It is a further object of the invention to provide a steam generating unit embodying a novel method of recirculating cooled products of combustion.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

The character of the invention, however, may be best understood by reference to one of its structural forms as illustrated by the accompanying drawings in which:

FIG. l is a vertical longitudinal View of a steam genice erating unit embodying the principles of the present invention;

FIG. 2 is a schematic view of the unit showing operation at high load, and

FIG. 3 is a schematic view of the unit showing operation at low load.

Referring first to FIG. l, which best shows the general features of the invention, the steam generating unit, indicated generally by the reference numeral 10, is shown as consisting of a furnace 11 and a boiler 12. The steam generating unit is mounted on a supporting structure 13. The furnace 11 is provided with a front wall 14, a rear wall 15, and side walls 16 defining a vertically-elongated combustion chamber 17. The furnace is of the type generally shown and described in the patent of Craig, #2,853,059, and is provided with a continuous slag bottom V18. The front wall 14 is provided with an abutment 19 which extends inwardly of the furnace; the abutment has an inclined upper surface 21 and a similarly inclined lower surface 22. The rear wall 15 is provided with a similar abutment 23 having an inclined upper surface 24 and an inclined lower surface 25. The inclined lower surfaces of the two abutments are provided with adjustable-vane, directional-flame, intertube burners 26 and 27 of the type shown in the patent to Craig, #2,759,460, which are connected in the usual way to sources of air and fuel. The abutments 19 and 23 extend in opposition to each other across the combustion chamber and divide the combustion chamber into a high-temperature cell 28 and an upper portion 29.

The boiler 12, which cooperates with the furnace 11 in the usual way, consists of a steam-and-Water drum 31 from which radiate the various tubes of the boiler system. Large downcomer tubes (not shown) carry the water to the lower portion of the boiler and this water flows upwardly through the system, particularly in water-wall tubes 20 which line the walls of the combustion chamber 17. Some of these water-wall tubes extend over the surfaces of the abutments 19 and 2.3. The directional-flame burners 26 and 27 are so arranged that the fuel and combustion air pass through gaps provided between certain of the water-wall tubes. Steam generated in the waterwall tubes 20 is introduced into the steam-and-water wall drum 31. At the upper part of the steam-and-water drum are located tubes 32 leading to a low-temperature convection superheater 33. The outlet end of the low-temperature superheater 33 is connected to the inlet side of a radiant superheater 30 mounted on the front wall 14. The outlet side of this superheater is connected to the inlet side of a high-temperature superheater 34, which is partially radiant and partially convective and whose outlet is connected to the turbine (not shown) in the usual manner.

The steam generating unit is provided with a back pass 35 divided by a wall 36 formed of tubes into a forward pass 37 and a rearward pass 38. The low-temperature superheater 33 is located in the rearward pass 38 and a low-temperature reheater 39 is located in the forward pass 37. At the upper portion of the combustion chamber 17 alternate tubes of the rearward wall 15 are bent forwardly to form a nose 41 defining a horizontal upper pass 42. A high-temperature reheater 43 resides in this upper pass as does the terminal convection portion of the high-temperature superheater 34. The major portion of the high-temperature superheater 34, however, resides in the upper portion of the combustion chamber 17 and is constructed in the form of large pendant platens. The lower ends of these platens extend below the nose 41 into the upper portion 29 of the combustion chamber.

At the lower end of the rearward pass 38 are located dampers 44 which control the flow of gas therethrough;

these dampers are connected to an actuating rod so as to operate together, the rod 45 being connected to a linear actuator, such as a hydraulic cylinder 46. Hydraulic lines 47 and 48 lead to a main control 49 and are connected to the cylinder. The said main control 49 is similar to that shown and described in the copending patent application of Parmakian, Serial Number 774,074, tiled November 14, 1958. In the lower end of the forward pass 37 are located dampers 51 which are movable together under the action of a rod 52 connected to a hydraulic cylinder 53 which operates under the control of hydraulic lines 54 and 55 connected to the main control 49.

The lower end of the back pass 35 of the steam generating unit is connected through an air heater 56 to a breaching 57 which leads to a stack (not Shown). The outlet of the high-temperature reheater 43 is connected to a reheated steam header 58, while the outlet of the high-temperature superheater 34 is connected to a superheated steam header 59.

The upper surface of the abutment 19 is formed with a series of gas openings formed by displacing waterwall tubes to form passages. A similar series of openings 80 is formed on the upper surface of the abutment 23. A large conduit 61 is connected to the gas openings on the abutment 19, while a similar conduit 62 is connected to the openings on the abutment 23. The conduits 61 and 62 are connected to a single conduit 65 having mounted therein a regulating damper 66. The end of the conduit 65 which is away from the connection to the conduits 61 and 62 is connected through a duct 60 having a damper 70 to the outlet of a fan 67. The inlet of the fan 67 is connected by a conduit 68 having a damper 95 to a portion of the back pass, such as the portion of the steam generating unit following an economizer 96 and lying between the dampers 44 and 51 on the one hand and the air heater 56 on the other hand.

The damper 66 is pivotally mounted and is actuated through rods 69 and 71, the latter rods being connected to a hydraulic cylinder 72, whose linear position is determined by the iluid pressure hydraulic lines '73 and 74 connected to the main control 49. The main control is connected through a line to a temperature-indicating device such as the thermocouple 76 located in the superheater steam header 59. The main control 49 is connected in a similar manner by a line 77 to a temperatureindicating device such as the thermocouple 78 lying in the reheated steam header 58.

Both the burner openings and the recirculated gas openings 50 and 80 are provided by bending back certain of the water-wall tubes 20. This is accomplished by bending four groups of three tubes back into a plane at a right angle to the surface of the front wall. A small number of tubes are left in the wall between the burners and between the gas openings. A typical burner is provided with a main fuel gun which extends centrally thereof and which is connected to a conduit from which it receives powdered coal and primary air. A gas gun is located centrally of the main fuel gun for maintaining ignition and an igniter is located in the burner closely adjacent the main fuel gun, the igniter being of the electric spark variety using gas as its fuel. Arranged along the burner above the gun are vanes whose inclination to the horizontal is adjustable by means of a control rod. In the lower part of the burner 26, vanes 87 are controlled by a rod 86. Air arrives at the burner through a duct 89. A typical recirculated gas opening 50 or 80 is provided with water tubes 20 which have been bent back to form a generally lozenge-shaped opening which is lined with refractory. Ducts 92 extend inwardly to each gas opening 50 on one side of the furnace from the main conduit 61, while the recirculated gas openings on the other side of the furnace are connected to the main conduit 62 by ducts 90, both of these last-named conduits extending parallel to the side and rear walls of the furnace outside of the line of air plenum chambers associated with the main fuel burners.

Connected to the duct 60 between the damper 70 and the duct 65 is a duct 97 which is provided with a damper 98. This damper is connected to a hydraulic cylinder 99 which is connected by lines 101 and 102 to the main control 49. The other end of the duct 97 is connected to the conduit 61.

A gas duct is connected at one end to the duct 68 between the damper 95 and the fan 67; the other end is connected to the back pass at a location upstream of the economizer 96 at an opening 106 in the rearward pass 38 under the low-temperature superheater 33. The duct 105 is provided with a damper 107 controllable by a cylinder 108 whose condition is determined by signals received from the main control 49 through lines 109 and 111. The damper 70 in the duct 60 is regulated by a cylinder 112 which is connected to the main control 49 by lines 113 and 114.

The upper portion of the low-temperature superheater 33 is connected to the radiant superheater 30 by a pipe 115, and in this pipe is located a spray-type desuperheater 116 which is connected for regulation to the main control 49 by a line 117. The vanes 87 of the burners 26 are connected by the rod 86 to a cylinder 118 which is regulated by the main control 49 through the medium of lines 119 and 121. The lower vanes 103 of the burners 27 are operated by a cylinder 122 which receives control signals from the main control 49 through connecting lines 123 and 124. The damper 95 in the duct 68 is adjusted by a cylinder 125 which is connected to the main control 49 by lines 126 and 127. The arrangement of burners 26 and 27 with their vanes, their cylinders 118 and 122, and the main control 49 provide for the flame placement principle of operation shown and described in the patent of Miller, No. 2,947,289, and the patent of Craig, No. 2,832,323.

The operation of the invention will now be readily understood in view of the above description. Fuel and air leave the burners 26 and 27 and enter the high-temperature cell 28 of the combustion chamber 17. The cell 28 exists at high temperature because of the restricted exit between the abutments 19 and 23 which causes the temperature in the portion 28 to reach a high value. Extreme turbulence takes place because of the opposed relationship of the burners and because of the presence in the cell of a pool of molten slag. A considerable portion of the combustion takes place in the cell 23 and, because of the high temperature, most of the formation of slag takes place there and is persuaded to stay there. The products of combustion pass upwardly through the combustion chamber 17 and a certain amount of combustion takes place in the upper portion 29. The gases leave the upper portion of the combustion chamber, ilow through the horizontal pass 42, and then flow downwardly through the back pass 35. The amounts of gas flowing through the forward portion 37 and the rearward portion 38 of the back pass are determined by the settings of the dampers 51 and 44. Gases pass through the air heater 56 into the breaching 57 and then low into the stack. Heat absorbed by the water-walls of the furnace causes the generation of steam; this steam enters the steam-and-water drum and, after suitable cleaning in the usual manner, passes downwardly into the lower portion of the low-temperature superheater 33, passing upwardly therethrough in a direction counter to the ow of gases. After being superheated in this manner to a certain degree, the steam then passes into the pipe where its temperature may be reduced by the desuperheater 116. At the other end of the pipe the steam enters the radiant superheater 30. From there the steam flows to the high-temperature superheater 34, entering the portion thereof adjacent the front wall 14 of the furnace 11. The steam in the high-temperature superheater is first subjected to a considerable amount of radiation because of the fact that a large part of this superheater is in the form of platens which extend deeply into the main combustion chamber 17. The steam is also subjected to a degree of convection superheating, passes into the superheated steam header 59, and flows from there to the high-pressure section of the turbine (not shown).

After passing through the high-pressure section of the turbine the steam returns to the boiler for reheating. It enters the lower end of the low-temperature reheater 39 and passes upwardly therethrough in counterflow to the ow of gases in the forward portion 37 of the back pass. Eventually, the steam reaches the high-temperature section 43 of the reheater, passes forwardly through the horizontal pass 42 in counteriiow to the flow of gases therethrough, and eventually enters the reheated steam header 58 for passage to the low-pressure section of the turbine.

The relationship between the superheat and the reheat temperatures can be controlled in the usual Way by regulating the portions of the total amount of products of combustion which pass through the forward portion 37 or the rearward portion 3S of the backpass. The regulation of dampers 34 and 51 accomplishes this purpose. It is not possible, however, to accomplish control of the absolute value of these variables by this means alone. By properly setting the damper 70 a certain amount of the products of combustion are carried back into the furnace. These products leave the back passes of the furnace through the conduits 68 and 105 under the impetus of the fan 67 and enter the conduit 60. They are then divided between the conduits 97 and 65 and enter the furnace either through the gas openings 5t) in the abutment 19 or through the gas openings 80 on the upper surface of the abutment 23. The introduction of large amounts of gas which have been recirculated in this manner has the effect of increasing the mass flow of gases over the heat exchange unit in the boiler. With respect to the convection sections of a boiler, an increase in mass flow (even though this increase is brought about by the introduction of lower temperature gases), results in a greater amount of heat transfer and a raising of the temperature of steam flowing within the convection elements. Ordinarily, of course, the recirculation of gas would be reserved only for those portions of load in which substantial increases in superheat and reheat are necessary. Let us suppose that the dampers 44, 51, and 70 are such that the preselected value of superheat is met at full load. if the load is lowered, that is to say, if the demands of the turbine for quantities of steam is reduced, the tiring rates of the burners 26 and 27 are also reduced. This means that there is less gas flow and lower gas temperatures throughout the furnace and boiler. The superheat will drop, and this will be indicated by the temperatureindicating thermocouple 76 which will transmit signals through the line 75 to the main control 49. Throttling the dampers together will not bring about a raise in reheat and superheat, so that it is necessary to make use of other means. The main control 49 senses that damper control will do nothing more to raise the reheat and superheat, so it operates through the hydraulic lines 113 and 114 to actuate the hydraulic cylinder 112. The cylinder operates through the connecting rods to reset the damper 70. The resetting in this case will be in the direction of opening the damper and causing a greater amount of recirculated gas to flow into the furnace. The increase in mass flow thus accomplished will raise both the reheat and the superheat. If one or the other of the two temperatures varies from the preselected value, the main control will operate through hydraulic lines 47, 48, 54, and 55 to move the hydraulic cylinders 46 and 53 to reset the dampers 44 and 51, respectively. For instance, if reheat is adequate but superheat is low, it might be necessary to open the damper 44 slightly and throttle the damper 51.

The introduction of the recirculated gas into the furnace by means of the gas openings 5t) and 80 has a number of unobvious advantages which make it particularly effective with the type of `furnace shown. Gases leaving the cell 28 of the combustion chamber are restricted to the area between the abutments 19 and 23 and ilow into the upper portion 29 of the combustion chamber along lines which are distinctly separated from the forward Wall 14 and the rearward wall 15 of the furnace. The main effect of this is that any slag particles which reside in the gas ow have a good opportunity to cool off and form hard particles before they strike any heat exchange surface. The tendency of this type of furnace is to keep the gases away from the front and rear walls until such cooling takes place. This separation or stratification of the gases away from the walls cooperates with the function of almost complete combustion in the cell Z8 to produce a very clean furnace. It has the effect also of providing for a greater uniformity of furnace temperature from side to side of the furnace and makes possible a smaller necessary furnace type. Now, when the reci:- culated gas is introduced through the gas openings in the upper surface of the abutments, the normal flow of gas through these openings will be upward, whereas the vanes of burners will be inclined generally downwardly into the high-temperature cell 28. The recirculated gas thus introduced through the openings 5t] and 80 passes upwardly above the abutments 19 and 23 and further accentuates the separation of the main body of the products of combustion from these walls. It might be said that the recirculated gases form an insulating blanket between the main products of combustion and the front and rear walls, thus providing the benefit of less slagging and fouling of the water walls. At the upper portion of the furnace the reversal of gas direction by the nose 41 and horizontal pass 42 produces sufficient mixing of the recirculated gases and the main product of combustion to accomplish the increases in mass flow control and through mixing of gases which are necessary to good control of reheat and superheat. As has been stated, in the high-temperature cell 28 combustion takes place almost entirely below the level of the abutments 19 and 23. Furthermore, the stratification of the recirculated gases adjacent the walls 14 and 15 assures that these gases will mix very little with the main products of combustion until they reach a point high in the combustion chamber. This means that combustion has a proper opportunity to take place with the proper mixture or fuel and air long before the recirculated gases are mixed with them.

Vthile the damper '70 in the duct tit) regulates the total amount of gas recirculated to the furnace, the settings of the damper 66 in the duct 65 and the damper 98 in the duct 97 determine the relative quantities of these gases which will pass through the gas openings 89 and 5t), respectively. The main control 49 operating through the control lines 73, 74, 101 and 102 assures that the settings of the dampers 66 and 98 are at all times oppositely complementary; that is to say, when the damper 66 is open, the damper 98 will be closed, and vice versa. The setting of the dampers 66 and 98 will be selected so that the temperature of superheated steam will be maintained constant, irrespective of load. Furthermore, at intermediate positions of one damper, the other damper will be in such a position that the total recirculated gas will be properly divided between the gas openings 50 at the front of the furnace and the gas openings S0 at the rear of the furnace.

Means is also provided for regulating the absolute temperature of the recirculated gases. The operation of the damper in the duct 66 and the damper 107 in the duct 105 determines the proportions of the gases will be hot (coming from the opening 196 above the economizer 96) and will be cool (coming from a portion of the apparatus below the economizer 96). There again, the operation of the dampers 95 and 107 are oppositely complementary, so that, when the damper 95 is wide open, the damper 107 will be closed, and vice versa; furthermore, at intermediate positions the settings of the dampers will be such that adequate cross-sectional flow capacity will be available in the two ducts 68 and 105 to satisfy the setting of the main control damper 70 in the reeirculated gas duct 60.

The header 59 leading to the turbine is provided with an orifice, provided, for instance, by a valve 12S. Lines 129 and 131 on either side of the valve are connected to the input side of the main control 49 and provide it with a signal indicative of load on the unit.

The operation of the desuperheater 116 by the main control 49 through the line 117 is well known and in the present apparatus is used to reduce the temperature of superheat when the steam value is higher than the predetermined design value, due to inaccuracies in other parts of the equipment.

In general, therefore, the apparatus provides for superheat control by adjusting the flame position from the front to the rear of the furnace by regulating the total amount of gas recirculated to the furnace, by regulating the relative amounts of recirculated gas passing into the furnace at burner level and at the nose, and by regulating the temperature of the recirculated gas. Furthermore, regulation of superheat control takes place by use of the desupergater 116 and by the proper use of the dampers 44 and At low load, which is the situation shown in FIG. 3, the vane settings of the burners is such as to cause the flame to be located adjacent the front wall of the furnace, which has the effect of giving high superheat by decreasing the residence time of the gases. At the same time, the total amount of gas recirculated will be large; that is to say, the setting of the damper 70 will be at its open position. Furthermore, the damper 98 will be closed and the damper 66 open to cause most of the gas to be introduced through the gas openings 80 or the abutment 23. The fact that the damper 66 is open will mean that the major portion of the recirculated gas will be introduced into the furnace at the opening 80, which will tend to push the main flow of products of combustion toward the front walls; this will tend to maintain the temperature of superheated steam in an upward direction and reduce the droop in the superheat curve that would normally be encountered at low load. A small quantitiy of gas will be permitted to pass into the furnace through the front gas openings 50 to provide for blanketing of the front and rear walls of the furnace. The large amount of recirculated gas has the effect of raising the temperature of superheat. The large amount of gas introduced through the gas openings 89 has the effect of accentuating the flame positioning of the burners; the flame is pushed toward the front Wall. It should be noted also that the positioning of the flame adjacent the front wall and adjacent the radiant superheater 30 and passing directly over the superheater 34 causes the superheat temperature to be high. Furthermore, by opening the damper 107 and closing the damper 95 the recireulated gas will be at a higher temperature, which means that, when the gas passes into the convection passes of the furnace, the effect will be to keep the superheat high.

At high load, the situation shown in FIG. 2 will prevail. The burner vane settings will be such that the flame will be positioned near the rear wall of the furnace, so that the gas from the flame will pass under the nose 41 and then flow into the convection passes. It will, therefore, be positioned as far from the radiant superheater 30 as possible and. will have a longer residence time in the furnace so that the gas temperature will be lower when it reaches the convection pass of the furnace. This is in absolute contrast to the situation shown in PEG. 3, where the llame takes the shortest route from the burners to the convection passes and, therefore, has the shortest residence time in the furnace whereby the gas temperature will be high when it reaches the convection passes. At high load the total amount of recirculated gas will be low, accomplished by a partial closure of the damper 70 in the duct 60. Furthermore, the division of the gases between the ducts 97 and the duct 65 will be such that most of the gases will be introduced into the furnace at the front gas openings 50 on the abutment 23; for this purpose the damper 98 will be open and the damper 66 closed. A small amount of gas will be permitted to flow through the gas openings to cool the tubes at that point and t0 protect the rear wall 15. The greater amount of gas will flow into the furnace through the gas openings 5t) and will push the llame (line of greatest mass ow of gas) further toward the rear wall. In this respect, the gas recirculation system reinforces the ame placement apparatus. At the same time, the settings of the dampers and 107 are such that the damper 107 would be closed, while the damper 95 will be open, so that those recircul ated gases which are introduced into the furnace at high load are at the lowest possible temperature. All of these factors, i.e., llame placement, amount of gas recirculated, and temperature of recirculated gas are selected so that at high load setting the characteristic curve of superheat versus load is low. In the preferred embodiment, the boiler is designed so that the characteristic curve with the high load setting will provide the desired design temperature at load; at loads over 100% the desuperlieater 116 will reduce the superheat temperature to the desired value.

Although in the above description it has been intimated that the high load setting would be used at loads above 100% load, and that at loads below 100% the low load setting would be used (thus requiring the desuperheating of a considerable amount of steam at intermediate loads), it will be understood that the main control 49 may operate the dampers through the hydraulic cylinders in such a manner that no desuperheating would be needed between 100% load and 50% load. The characteristic curve would be any one of a family between the two curves. For instance, at 75% load, there is a curve having the same sloping characteristic as the two curves, which curve will pass through the desired superheat temperature line at 75% load. The main control 49 will select the proper settings of the elements to provide the particular characteristic curve needed for a particular load to obtain such a crossing. In other words, as the load is dropped from 100% load to 50% load, the settings of the vanes of the burners 26 and 27 and the division of gas between the openings 50 and 80 will be continuously adjusted from the positions which cause the flame to be positioned near the front wall of the furnace to a position where the flame rises along the rear wall, the flame at intermediate loads occupying intermediate positions between the front and rear walls. In the same way, as the load goes from 100% load to 50% load, the damper 70 (which controls the total amount of gas recirculated) will be continuously adjusted from a closed position at 100% load to an open position at 50% load; at intermediate loads the damper 70 will occupy intermediate positions between open and closed. in the same way, as the load is moved from 100% load to 50% load, the settings of the dampers 66 and 98 will be moved from a condition in which the damper 66 is closed and the damper 98- is wide open (at 100% load) to a conditi on where the damper 66 is open and the damper 98 is closed (at 50% load); at intermediate loads, the settings of the dampers will be at intermediate positions proportionate to the particular load. In a similar manner, as the load is adjusted from 100% load to 50% load, the dampers 95 and 107 (which regulate the temperature of the recirculated gas) will be adjusted from a position where the damper 95 is wide open and the damper 107 is closed (at high load) to a condition where the damper 95 is closed and the damper 107 is open (at low load); at intermediate loads, the dampers will be at intermediate positions to give intermediate temperatures of recirculated gas commensurate with the particular load at any given time.

Certain minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly corne within the scope claimed.

The invention having been thus described, what is new and desired to secure by Letters Patent is:

1. A steam generating unit, comprising (a) front, rear, and side Walls defining a verticallyelongated combustion chamber,

(b) a nose extending from the rear Wall at the upper portion thereof,

(c) a convection pass opening into the rear wall above the nose,

(d) opposed abutments extending toward one another from the front and the rear walls, the abutments dening a high temperature cell in the lower portion of the combustion chamber,

(e) fuel-burning means located on downwardly-directed surfaces of the abutments for producing a flame which extends upwardly between the abutments,

() a iirst means associated with the fuel-burning means to position the said flame relative to the front and rear walls,

(g) gas openings located on upwardly-directed surfaces of the abutments,

(h) and second means for introducing gases to the openings to reinforce the said rst means in its function of positioning the said flame relative to the front and rear walls, the second means consisting of a main control which receives signals indicative of load and of the temperature of superheated steam and sends out signals to a rst control apparatus to regulate the total amount of recirculated gas in response to load and to a second control apparatus to regulate the proportions by which the said gas is divided in response to superheat temperature between the gas openings of the front wall abutment and the gas openings of the rear Wall abutment.

2. A steam generating unit as set forth in claim l, wherein a radiant superheater is provided in the upper part of the furnace adjacent the front wall opposite the said nose.

3. A steam generating unit as recited in claim 1, wherein the combustion chamber is provided with roof and bottom walls and the nose defines with the roof wall the convection pass containing a convection superheater, wherein the fuel-burning means consists of directionalflame inter-tube burners, wherein the gas openings are of the inter-tube type, and wherein the second means introduces re-circulated gases to the openings with a horizontal flow component so that these gases reinforce the said first means in its function of positioning the said flame relative to the front and rear walls by striking the flame and pushing it in the desired direction.

References Cited in the le of this patent UNITED STATES PATENTS 2,798,464 Seidl July 9, 1957 2,947,289 Miller Aug. 2, 1960 2,973,750 Armacost Mar. 7, 1961 

1. A STEAM GENERATING UNIT, COMPRISING (A) FRONT, REAR, AND SIDE WALLS DEFINING A VERTICALLYELONGATED COMBUSTION CHAMBER, (B) A NOSE EXTENDING FROM THE REAR WALL AT THE UPPER PORTION THEREOF, (C) A CONVECTION PASS OPENING INTO THE REAR WALL ABOVE THE NOSE, (D) OPPOSED ABUTMENTS EXTENDING TOWARD ONE ANOTHER FROM THE FRONT AND THE REAR WALLS, THE ABUTMENTS DEFINING A HIGH TEMPERATURE CELL IN THE LOWER PORTION OF THE COMBUSTION CHAMBER, (E) FUEL-BURNING MEANS LOCATED ON DOWNWARDLY-DIRECTED SURFACES OF THE ABUTMENTS FOR PRODUCING A FLAME WHICH EXTENDS UPWARDLY BETWEEN THE ABUTMENTS, (F) A FIRST MEANS ASSOCIATED WITH THE FUEL-BURNING MEANS TO POSITION THE SAID FLAME RELATIVE TO THE FRONT AND REAR WALLS, (G) GAS OPENINGS LOCATED ON UPWARDLY-DIRECTED SURFACES OF THE ABUTMENTS, (H) AND SECOND MEANS FOR INTRODUCING GASES TO THE OPENINGS TO REINFORCE THE SAID FIRST MEANS IN ITS FUNCTION OF POSITIONING THE SAID FLAME RELATIVE TO THE FRONT AND REAR WALLS, THE SECOND MEANS CONSISTING OF A MAIN CONTROL WHICH RECEIVES SIGNALS INDICATIVE OF LOAD AND OF THE TEMPERATURE OF SUPERHEATED STEAM AND SENDS OUT SIGNALS TO A FIRST CONTROL APPARATUS TO REGULATE THE TOTAL AMOUNT OF RECIRCULATED GAS IN RESPONSE TO LOAD AND TO A SECOND CONTROL APPARATUS TO REGULATE THE PROPORTIONS BY WHICH THE SAID GAS IS DIVIDED IN RESPONSE TO SUPERHEAT TEMPERATURE BETWEEN THE GAS OPENINGS OF THE FRONT WALL ABUTMENT AND THE GAS OPENINGS OF THE REAR WALL ABUTMENT. 